1) Field of the Invention
The present invention relates to an ambient field level monitor device applicable suitably for a radio mobile terminal or station.
2) Description of the Related Art
In the recent years, with an explosive increase of portable telephone subscribers, communication enterprises installs a large number of base stations and increase the number of channels to be put to use, thereby attempting to stabilize communications.
Viewing a channel detection method taken in a Japanese PDC (Personal Digital Cellular) system, a radio mobile terminal, forming a mobile station, detects the level of a carrier from a base station within a peripheral cell during an idle slot time to measure a value of its received signal strength intensity (which will be referred hereinafter to as an RSSI), and reports the measurements to the base station, while the base station receives that report successively. During communications with the mobile station, if a-peripheral cell exists which shows a received level higher than that of the current cell by a given value or more, the base station refers to a base station in that peripheral cell for the presence or absence of a free channel and, if the free channel is present, transmits channel information to the mobile station (which will sometimes be referred hereinafter to as mobile equipment). Thus, the mobile station comes to use that channel.
This RSSI value detection method is made to obtain an intermediate frequency signal down-converted from a radio frequency signal. That is, while the intermediate frequency signal is inputted to a limiter to undergo amplitude limiting processing, a signal is taken out from a predetermined portion of that limiter and then processed in another circuit, thereby obtaining the RSSI value. The limiter is composed of a plurality of logarithmic amplifiers or the like cascaded, and the intermediate frequency signal taken out from the predetermined portion is smoothed through a low-pass filter in another circuit to detect a DC level so that the detection value is outputted as an RSSI value.
FIG. 18 is an illustration of a cell disposition in the Japanese PDC system and shows a base station disposition. Circles forming cells A to F represent schematic zones the base stations can cover respectively. In FIG. 18, in case where a mobile station 60 first stays in the zone D and then moves into the zone B, the mobile station 60 has used a channel pertaining to the zone D at first, and then gradually approaches the zones C and F and finally reaches the zone B. In this instance, in order to ensure stable communications under such an environment, there is a need for the mobile station 60 to certainly capture, of a plurality of receivable carriers from the base stations, a channel showing the highest RSSI value as occasion calls. Thus, the mobile station 60 is required to make measurements of the RSSI values in the peripheral zones with high accuracy during a period of time for an idle slot.
This idle slot is one of TDMA slots, while FIG. 19 is an illustration of a TDMA slot configuration on the mobile station 60 side. In FIG. 19, the mobile station 60 side TDMA slot is made up of four types of slots: a reception slot R, an idle slot I, a transmission slot T and a control slot LM. The mobile station 60 conducts the reception through the reception slot R and receives carriers from a plurality of other base stations through the idle slot I to sense the presence or absence of a free channel the mobile station 60 itself can transmit (carrier sense), while performing the transmission through the transmission slot T and measuring a level for the antenna switching diversity control through the control slot LM. As one of the countermeasures against the fading, the mobile station 60 employs the antenna diversity for switching between usually two receiving antennas to receive a signal from one of the two antennas which exhibits the best receiving condition. That is, during this LM time period, the mobile station 60 measures the RSSI level through each of the two antennas to select one antenna indicating a higher level.
For conducting the carrier sense during the slot I time period, the mobile station 60 switches among the channels by altering the frequency value of a PLL (Phase Locked Loop; referred to as a PLL) frequency synthesizer, thereby measuring the RSSI value at every channel. Meanwhile, when the mobile station 60 makes the measurement of the RSSI value, owing to the influence of the two facts: {circle around (1)}the received level variation caused by the fading and {circle around (2)}the RSSI value sampling error occurring because the level detection is made in a non-synchronous (asynchronous) condition, the measurement result becomes unstable and its accuracy drops. The variation due to the fading stems from the fact that the received level varies momently depending on the situation of the transmission path. Further, the RSSI value sampling error originates from the fact that the amplitude of the reception signal varies at all times. With reference to FIG. 20, a description will be given hereinbelow of this error.
FIG. 20 is an illustration of an RSSI detection voltage waveform appearing at the application of a capacitance value of 100 pF. This waveform obtained by giving a capacitance value of 100 pF, shows the case where an xe2x80x9call-1xe2x80x9d modulated wave is received, where a peak appears at an interval of symbol clock time (the inverse number of 21 kHz) along a time base and its amplitude varies at all times. Further, if the sampling is made to obtain RSSI value at the sampling timings t1, t2 and t3 indicated in FIG. 20, a variation of the measured level depending upon the sampling timings is large so that the RSSI value detected contains errors and becomes unstable. Accordingly, the operation of the mobile station 60 itself also becomes unstable by conducting the channel switching operation on the basis of the information on the RSSI value in a peripheral zone.
A factor to vary the signal amplitude at all times is that the transmission is made through a Nyquist filter on the transmission side. That is, the base station imposes limitation in band on a transmitting signal through the use of the Nyquist filter to eliminate the interference between codes in a receiver. Thus, at the transmission filter output in a transmitter, filter responses with symbol waveforms, each generated at every symbol, overlap with each other so that the signal amplitude of the Nyquist filter output becomes constant at the symbol point, whereas the amplitude goes high or low at the other points. For this reason, the locus of the modulated signal on a phase plane lies on a unit circle at the symbol point, but shifts from the unit circle at the other points. The shifting from this unit circle will be described hereinbelow with reference to FIGS. 21A to 21C.
FIG. 21A is an illustration of a disposition of reception signal symbol points on the receive side. In FIG. 21A, an amplitude locus connecting four symbol points 61a, 61b, 61c and 61d shifts outwardly or inwardly from a unit circle because of the effect of the interposition of a route Nyquist filter on the transmission side. Further, FIG. 21B is an illustration of an RSSI amplitude detection values at points in a phase synchronous condition (phase synchronized condition) In FIG. 21B, since the detection timings (corresponding to arrows from the origin) coincide with the aforesaid four symbol points 61a, 61b, 61c and 61d, the detection amplitudes are constant between each detection point of the RSSI value. On the other hand, FIG. 21C is an illustration of an RSSI amplitude detection values at points in a phase non-synchronous condition (not phase synchronized condition), where the detection amplitude of the RSSI value varies in accordance with the measurement timing because the detection timings (corresponding to arrows from the origin) shift from the symbol points.
Therefore, so far, a capacitor with a large capacitance has been added to an RSSI detection circuit. In this case, the time constant of its low-pass filter increases to cause the smoothing time of the low-pass filter to be prolonged, and the integration of the amplitude variation due to the shifting of the RSSI detection timing reduces the measurement error.
Japanese Patent Laid-Open (Kokai) No. HEI7-240697 (which will be referred hereinafter to as a publication 1) discloses a technique for switching the time constant within a PLL frequency synthesizer circuit.
This publication 1 discloses a technique relating to a frequency synthesizer circuit, without enlarging the scale of a circuit, capable of stabilizing the transmission/reception performance in a digital mobile communication system or the like. This technique produces a variation of the gain of a loop filter in the PLL frequency synthesizer circuit in order to satisfy the requirements for a high-speed response performance with respect to a carrier for the carrier sense and for a high C/N ratio with respect to a carrier for the reception.
Concretely, according to the technique disclosed in this publication 1, the time constant of a time constant circuit in the PLL frequency synthesizer circuit is altered in accordance with the response times required individually for the communication slots so that a voltage control oscillating means is controllable. Thus, without providing a PLL frequency synthesizer circuit for each of the communication slots, the supply of an oscillating frequency signal with a stable and sufficient C/N ratio to a transmission/reception circuit becomes feasible, thereby enabling the significant improvement of the transmission/reception performance of the digital mobile communication system or the like.
More concretely, during the slot I time period, for the purpose of quick carrier sense, even if sacrificing the C/N ratio somewhat, the high-speed response performance is preferentially secured so that the rising of the voltage waveform in the output of the voltage control oscillator becomes quickened. In addition, for quickening this rising, control is implemented to decrease the time constant of the tuning voltage to be inputted to the voltage control oscillator.
Moreover, Japanese Patent Laid-Open (Kokai) No. HEI7-202827 (which will be referred hereinafter to as a publication 2) discloses an approach to improve the accuracy of the RSSI detection value in a phase synchronous condition.
This publication 2 discloses the following technique concerning a received level detection circuit to be employed for a digital wireless terminal based on the time-division multiplexing. That is, according to this technique, a reception signal composed of a burst waveform in the digital wireless terminal is led to two low-pass filters different in time constant from each other, and in synchronism with a timing of the burst waveform of the reception signal, the reception level output from the low-pass filter with a higher time constant is selected for the burst time period while the reception level output from the low-pass filter with a lower time constant is selected for a time period other than the burst time period, with the result that the detection level at a detection point within the burst time period is securable with high accuracy and the detection level at a detection point within the time period other than the burst time period is also securable with high accuracy.
However, the above-mentioned publication 1 does not disclose a technique for improving the accuracy of the detection of the RSSI value. Further, the technique disclosed in the above-mentioned publication 2 involves switching the low-pass filters in synchronism with a timing of a burst waveform, but not teaching a technique to be taken in the asynchronous condition with that timing.
In addition, in the case of the technique in which a capacitor with a large capacitance is incorporated into the RSSI detection circuit as mentioned above, increasing the constant of a capacitor to be incorporated into the RSSI detection circuit causes an increase in the time constant of the low-pass filter, which interferes with the RSSI measurements to be made by two antennas during the control slot LM time period to produce the drawbacks such as a diversity malfunction, an decrease in the diversity gain and a decrease in the RSSI detection value. For this reason, there is a limit to the time constant to be set by the incorporation of the capacitor or the like.
Accordingly, the present invention has been developed with a view to eliminating these problems, and it is therefore an object of this invention to provide an ambient field level monitor device for a radio mobile terminal which is made to, when being out of synchronism with a phase of a received radio signal, switch an RSSI time constant to a value greater than an RSSI time constant to be taken in a synchronous condition with the phase of the radio signal, thereby lessen the measurement error stemming from the shifting of the RSSI sampling timing, making the RSSI value detection stable and enhancing the detection accuracy.
For this purpose, in accordance with this invention, there is provided an ambient field level monitor device for a radio mobile terminal, comprising a reception section for receiving a radio signal arriving at the radio mobile terminal to output a reception signal originating from the received radio signal and further to output an ambient field level signal, a synchronization decision section for receiving the reception signal from the reception section to judge whether or not the reception section is in synchronism with the radio signal, and a time constant variable type ambient field level monitoring section for receiving the ambient field level signal from the reception section, wherein, if the synchronization decision section judges that the reception section is out of synchronism with the radio signal, the ambient field level monitoring section increases its own time constant to exceed its own time constant to be taken for when the synchronization decision section judges that the reception section is in synchronism with the radio signal and if it judges that the reception section is synchronism with the radio signal, the ambient field level monitoring section decreases the time constant.
Thus, without exerting influence on the RSSI measurement in a synchronous condition, the reduction of the RSSI level measurement error becomes feasible in a non-synchronous condition. Whereupon, a proper zone selection is allowed to the radio mobile terminal to prevent the interruption of communications. Additionally, since there is no need to increase the time constant value in a synchronous condition with the radio signal, it is possible to eliminate the problems such as the diversity malfunction and the diversity gain reduction, and further to remove the troubles such as a decrease in the detected RSSI value.
Furthermore, in this case, it is also appropriate that the time constant variable type ambient field level monitoring section is constructed by a combination of a capacitor and a resistor, and constructed in the form of an analog integrating circuit in which any one of the capacitor and the resistor is made such that its impedance value is variable, or that the time constant variable type ambient field level monitoring section is made up of an analog/digital converter for performing an analog-to-digital conversion of the ambient field level signal forming an analog signal from the reception section, a digital integrating circuit for integrating a digital signal from the analog/digital converter, with the digital integrating circuit being made such that its integrating state is variable, and a digital/analog converter for conducting a digital-to-analog conversion of the digital signal from the digital integrating circuit.
In this way, similarly, without affecting the RSSI measurement in a synchronous condition, it is possible to lessen the RSSI level measurement error of a peripheral zone in a non-synchronous condition.
Still further, it is also acceptable that the ambient field level monitoring section is made to switch its time constant or digital sampling data extracting interval or mode for a predetermined period of time from receiving a channel switching signal through the use of the channel switching signal.
Similarly, stable and high-accuracy RSSI measurement becomes feasible in this way.
Moreover, in accordance with this invention, there is provided an ambient field level monitor device for a radio mobile terminal, comprising a reception section for receiving a radio signal arriving at the radio mobile terminal to output a reception signal originating from the radio signal and further to output an ambient field level signal, a synchronization decision section for receiving the reception signal from the reception section to judge whether or not the reception section is in synchronism with the radio signal, and an ambient field level monitoring section for receiving the ambient field level signal from the reception section and for extracting a digital sampling data from the ambient field level signal to detect an ambient field level, wherein, for the detection of the ambient field level, the ambient field level monitoring section selectively employs different modes for the extraction of the digital sampling data from the ambient field level signal for when the synchronization decision section judges that the reception section is out of synchronism with the radio signal and for when it judges that the reception section is in synchronism with the radio signal.
Thus, without exerting influence on the RSSI measurement in a synchronous condition, the reduction of the RSSI level measurement error becomes feasible in an asynchronous condition, and a proper zone selection is allowed to the radio mobile terminal to prevent the interruption of communications. Additionally, it is possible to eliminate the problems such as a diversity malfunction and a diversity gain reduction, in a synchronous condition with the radio signal, and further to prevent the troubles such as a decrease in the RSSI detection value.
In addition, it is also appropriate that the ambient field level monitoring section extracts digital sampling data at a data varying point of the ambient field level signal when the synchronization decision section judges that the reception section is out of synchronism with the radio signal, or that the ambient field level monitoring section extracts high-speed digital sampling data from the ambient field level signal when the synchronization decision section judges that the reception section is out of synchronism with the radio signal.
In this way, it is possible to detect a precise RSSI value hardly containing errors, and further to stable make the RSSI measurement with high accuracy. Additionally, without affecting the RSSI measurement in a synchronous condition, it is possible to reduce the error in the RSSI level measurement of a peripheral zone in an asynchronous condition.
Besides, it is also acceptable that the ambient field level monitoring section changes its time constant or digital sampling data extraction interval or mode for a predetermined period of time from receiving a channel switching signal through the use of a channel switching signal. Similarly, this enables the stable and high-accuracy RSSI measurement.